This invention relates to a cable data system by which two-way data communications, such as Internet access, is provided via a cable televisions system. In particular, this invention relates to an improved method and apparatus for connecting the physical cable media to a cable modem termination system (CMTS) so as to expedite installation and provide a more serviceable CMTS to maintenance personnel.
Cable television systems are well known. In such a system, several different frequency-division multiplexed television channels are distributed to subscribers over a coaxial cable. Each television channel is typically allocated a frequency band, (typically 6 MHz.) in which audio and video information for a television channel is carried. Data signals can also be modulated onto an RF carrier and be transmitted in one or more of the pre-allocated television frequency bands. By allocating one or more T.V. channels for data, the cable television network can readily carry data, such as the data exchanged between computers. Cable data systems provide Internet access to subscribers at speeds that are far greater than dial-up modems.
A cable communications system topology resembles an inverted tree or a directed acyclical graph. The top or upper-most node in a directed acyclical graph (DAG) representing a cable distribution system is the node from which signal information is distributed and is frequently referred to as the cable system head end. Each link in the DAG represents a coaxial cable on which there might be several different frequency-division multiplexed signals.
One or more cable modem termination systems (CMTS) at or near the head end direct the distribution and collection of data to, and from, cable data system subscribers. At the head end of a cable data system, there are typically hundreds of physical cables that branch out from the head end to and from the system subscribersxe2x80x2 homes over a hybrid-fiber coaxial system. Downstream signals are transported on a hybrid-fiber coaxial cable with carrier frequencies centered above the 50 MHz point in the cable spectrum, while upstream signals are transported on a hybrid-fiber coaxial cable with carrier frequencies centered in the 5-42 MHz region of the cable spectrum.
Several different upstream channels can be frequency division multiplexed onto a single cable. In order to recover each channel, the upstream signal must be divided (or split) so that it can be coupled into separate RF band pass filters before being terminated at a unique Physical Interface (PHY) chip. Each PHY chip filters and demodulates the upstream signal for a particular channel to re-create the digital data stream for that channel.
If each upstream channel is transported on a different upstream cable, then a different cable must be used to inject each upstream channel into a unique CMTS upstream connector. In the high-capacity, high-bandwidth CMTS systems of the future, there will be many upstream channels supported by a single CMTS, so this will result in a large number of cables connecting to the CMTS in a relatively small area (yielding very high cable densities). These high cable densities can be difficult to manage.
However, if several upstream channels are frequency-division multiplexed on a single upstream cable, then it is possible to reduce the high cable densities at the CMTS by injecting multiples of these upstream channels over a single cable into a single CMTS upstream connector. For example, if four upstream channels are always frequency-division multiplexed on every upstream cable, then a system that originally required 100 separate upstream cables could be re-cabled using only 25 upstream cables, where each of the upstream cables carries four upstream channels into the CMTS. However, in order to permit this desirable decrease in cable density, the CMTS must be able to provide the further splitting required to route each of the upstream channels to a unique CMTS RF band pass filter and PHY chip. Since the actual number of upstream channels that will be multiplexed together is oftentimes unknown until the cable system is being assembled, the amount of integrated splitting provided by the CMTS must be configurable by the installer. This can be accomplished by providing several circuit cards with different splitting ratios so that the CMTS equipage can be fine-tuned to match the needs of the installer.
Although this solution solves the cable density problem, it leads to another problem, because installation of circuit cards with different splitting ratios will lead to different amounts of loss in the upstream signal paths. To maintain acceptable signal levels at the PHY chips, the CMTS should probably provide suitable gain to compensate for the splitting loss. Unfortunately, the required gain cannot be predicted until the installer has inserted the actual splitter circuit card (with a particular splitting ratio pre-defined on the circuit card). Thus, it would be beneficial if the CMTS could sense the splitting ratio on the installed splitter circuit card and provide adequate gain to directly compensate for the resulting splitter loss.
In a cable data system, upstream frequency-division multiplexed signals that received at a CMTS on a single cable are divided (i.e. split) using an analog radio frequency (RF) splitter so that individual frequency bands can be selectively filtered by CMTS channel interface cards that include band pass filters, which in at least one embodiment are programmably tuned to the center frequency of a particular pass band.
The CMTS signal splitter card is inserted into a card edge connector, preferably at the rear of the CMTS and includes on the card, one or more resistive networks which, by way of card edge connector wiring and splitter card wiring, automatically identifies to the CMTS and the channel interface cards (which filter and amplify signals in predetermined frequency bands) the signal splitting ratio of the signal splitter card.
A known artifact of splitting an RF cable signal, using an analog signal splitter, is a corresponding reduction in the split signal amplitude that is proportional to the signal division factor. By way of example, splitting an RF signal into two (2) separate signals causes each of the two, separate output signals from the splitter to be 3 dB (decibels) below the level of the signal input to the splitter. Splitting a signal into four (4) signals causes each one of the four separate outputs to be 6 dB below the signal level input to the splitter. Splitting a signal into eight (8) signals causes each one of the eight separate outputs to be 9 dB below the signal level input to the splitter.
In the preferred embodiment, the CMTS channel interface cards recognize the division ratio of the signal splitter card in the CMTS back plane and automatically increase the gain of programmable gain compensation stage (i.e. an amplifier) which is preferably an amplifier that is either band-pass tunable, or having a wide enough frequency response to amplify the entire spectrum of signals that it will be expected to amplify, so as to restore the signal amplitude output from the splitter to the amplitude that was input to the splitter. Different signal splitter cards with different signal division factors use different resistive indicator networks to identify to the CMTS and the channel interface cards, the amount of amplification required to compensate for the signal splitting. Increased system reliability is achieved by automatically adjusting RF signal amplification after splitting a frequency-division multiplexed signal into its respective components for subsequent recovery of data signals from subscribers.